How human activities produce greenhouse gases
Most important human activities emit greenhouse gases (GHGs). Emissions started to rise
dramatically in the 1800s due to the Industrial Revolution and changes in land use. Many greenhouse
gas-emitting activities are now essential to the global economy and form a fundamental part of modern
Carbon dioxide from the burning of fossil fuels is the largest single source of greenhouse gas
emissions from human activities. The supply and use of fossil fuels accounts for about 80 percent
of mankind's carbon dioxide (CO2) emissions, one fifth of the methane
(CH4), and a significant quantity of nitrous oxide (N2O). It also produces
nitrogen oxides (NOx), hydrocarbons (HCs), and carbon monoxide (CO), which, though not
greenhouse gases themselves, influence chemical cycles in the atmosphere that create or destroy other
greenhouse gases, such as tropospheric ozone. Meanwhile, fuel-related releases of sulphate aerosols
are temporarily masking part of the warming effect of greenhouse gases.
Most emissions associated with energy use result when fossil fuels are burned. Oil, natural
gas, and coal (which emits the most carbon per unit of energy supplied) furnish most of the energy
used to produce electricity, run automobiles, heat houses, and power factories. If fuel burned
completely, the only by-product containing carbon would be carbon dioxide. But combustion is often
incomplete, so carbon monoxide and other hydrocarbons are also produced. Nitrous oxide and other
nitrogen oxides are produced because fuel combustion causes nitrogen in the fuel or air to combine
with oxygen in the air. Sulphur oxides (SOx) result when sulphur (primarily from coal and
heavy fuel oil) combines with oxygen; the resulting sulphate aerosols have a cooling effect on the
Extracting, processing, transporting, and distributing fossil fuels also releases greenhouse
gases. These releases can be deliberate, as when natural gas is flared or vented from oil wells,
emitting mostly carbon dioxide and methane, respectively. They can also result from accidents, poor
maintenance, and small leaks in well heads, pipe fittings, and pipelines. Methane occurring naturally
in coal seams as pockets of gas or "dissolved" in the coal itself is released when coal is
mined or pulverized. Hydrocarbons enter the atmosphere as a result of oil spills from tanker ships or
small losses during the routine fueling of motor vehicles.
Deforestation is the second largest source of carbon dioxide. When forests are cleared for
agriculture or development, most of the carbon in the burned or decomposing trees escapes to the
atmosphere. However, when new forests are planted the growing trees absorb carbon dioxide, removing
it from the atmosphere. Recent net deforestation has occurred mainly in the tropics. There is a great
deal of scientific uncertainty about emissions from deforestation and other land-use changes, but it
is estimated that from 800 million to 2.4 billion tonnes of carbon are released globally every year.
Producing lime (calcium oxide) to make cement accounts for 3% of CO2 emissions from
industrial sources. Like the CO2 emitted from fossil fuels, the carbon dioxide
released during cement production is derived from limestone and is thus of fossil origin, primarily
sea shells and other biomass buried in ancient ocean sediments.
Domesticated animals emit methane. The second-most important greenhouse gas after carbon
dioxide, methane is produced by cattle, dairy cows, buffalo, goats, sheep, camels, pigs, and horses.
Most livestock-related methane emissions are produced by "enteric fermentation" of food by
bacteria and other microbes in the animals' digestive tracts; another source is the decomposition
of animal manure. Livestock account for 30% of the methane emissions from human activities.
Rice cultivation also releases methane . . . "Wetland" or "paddy" rice
farming produces roughly one-fifth to one-quarter of global methane emissions from human activities.
Accounting for over 90 percent of all rice production, wetland rice is grown in fields that are
flooded or irrigated for much of the growing season. Bacteria and other micro-organisms in the soil
of the flooded rice paddy decompose organic matter and produce methane.
. . . as does the disposal and treatment of garbage and human wastes. When garbage is buried
in a landfill, it sooner or later undergoes anaerobic (oxygen-free) decomposition and emits methane
(and some carbon dioxide). Unless the gas is captured and used as a fuel, the methane eventually
escapes to the atmosphere. This source of methane is more common near cities, where garbage from many
homes is brought to a central landfill, than in rural areas where garbage is typically burned or left
to decompose in the open air. Methane is also emitted when human waste (sewage) is treated
anaerobically, for example in anaerobic ponds or lagoons.
Fertilizer use increases nitrous oxide emissions. The nitrogen contained in many mineral and
organic fertilizers and manures enhances the natural processes of nitrification and denitrification
that are carried out by bacteria and other microbes in the soil. These processes convert some
nitrogen into nitrous oxide. The amount of N2O emitted for each unit of nitrogen applied
to the soil depends on the type and amount of fertilizer, soil conditions, and climate - a complex
equation that is not fully understood.
Industry has created a number of long-lived and potent greenhouse gases for specialized uses.
Developed in the 1920s, chlorofluorocarbons (CFCs) have been used as propellants in aerosol cans, in
the manufacture of plastic foams for cushions and other products, in the cooling coils of
refrigerators and air conditioners, as fire extinguishing materials, and as solvents for cleaning.
Thanks to the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer, atmospheric
concentrations of many CFCs are stablizing and expected to decline over the coming decades. Other
halocarbons that are being used as ozone-safe replacements for CFCs – notably
hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) – contribute to global warming and so are
targeted for reduction under the 1997 Kyoto Protocol. The Protocol also targets sulphur hexafluoride
(SF6), used as an electric insulator, heat conductor, and freezing agent; molecule for
molecule, its global warming potential is thought to be 23,900 times greater than that of carbon